Numerous systems for removing pollutants from boiler plant flue gases are known in the art. The need for such systems has become particularly acute in recent years, as environmental concerns have become more prevalent.
Prior art on the chemistry of the manufacture of acids of sulphur and their influence on oxides of nitrogen is very old, dating back to 1797 in the United States with John Harrison's manufacture in Philadelphia. The theories and practice are described in "The Manufacture of Sulphuric Acid" (W. Duecker and J. West eds. 1959).
The chemical reactions involved in this patent application are described on pages 178 and 694 of F. Ephram, "Inorganic Chemistry" (P. Thorne and E. Roberts 4th ed. 1943). U.S. Pat. No. 3,920,421 further reflects the general knowledge regarding the interaction of sulphur dioxides and nitric oxides, but no known prior art recovered merchantable products of sulphuric acid and sulphur dioxide and ammonium salts therefor while simultaneously recovering flue gas heat in boiler feedwater.
Among the prior art systems are those which employ heat exchanger apparatus to cool flue gases and collect the condensate, which contains pollutants from the gases. By removing the condensate, pollutants are also removed. Examples of such techniques are seen in U.S. Pat. No. 4,876,986 to Johnson, which employs rotating rock beds to transfer heat, and U.S. Pat. No. 4,597,433 to Johnson, which employs large size pebble beds to transfer heat.
Many of the prior art systems were developed in response to a demand for increased energy efficiency. These systems were primarily concerned with the heat exchange processes, rather than the pollutant removal processes. This segment of the art is reflected in a series of patents issued to Mr. Donald Warner and assigned to Heat Exchanger Industries, Inc., including U.S. Pat. Nos. 4,487,139, 4,526,112, and 4,557,202. These patents recognize and utilize the technique of condensing acidic pollutants, especially sulphuric acid (H.sub.2 SO.sub.4), from the flue gas.
Condensation of H.sub.2 SO.sub.4 occurs as a result of the combination of water and sulphur trioxide, according to the following equation: EQU H.sub.2 O+SO.sub.3 .fwdarw.H.sub.2 SO.sub.4
SO.sub.3 is a well-known and highly effective desiccant, and as long as water is present the two will combine and, if cooled, condense. Flue gas typically contains SO.sub.2 as well as SO.sub.3, however, and SO.sub.2 does not readily combine with H.sub.2 O. It has therefore been a further practice in the art to oxidize the SO.sub.2 pollutants to SO.sub.3 for subsequent removal by condensation. This procedure is shown for example, by U.S. Pat. No. 4,910,011.
Prior art systems that remove pollutants by heat exchange and condensation also do not address the effect of particulate pollutants, such as fly ash, on the oxidization and removal of sulphuric pollutants. Some references employ sprays of water or a large amount of condensation, as in U.S. Pat. No. 4,487,139, to wash particulate matter from the heat exchanger and keep the exchanger in efficient operating order. However, none of the references appreciates the deleterious effect of fly ash and other poisonous particulates on the oxidization process, or provides a method of effectively and efficiently removing pollutants by heat exchange without comprised performance.
Although it is known in the prior art that flue gas condensation contains pollutants, and that causing such condensation is one way to remove pollutants, an effective system has not been developed which both removes substantially all pollutants and is capable of being readily retrofit onto existing power plant systems.
It is therefore a general object of the invention to provide a more efficient heat exchange between boiler plant flue gas and boiler feedwater flowing through tubes disposed in the flue gas.
A further objective is to achieve a more perfect removal of sulphur and nitrogen oxides, as well as aerosols and particulates, from the flue gas.
An additional object of this invention is to provide a method of removing pollutants in which the oxidization of SO.sub.2 to SO.sub.3 is uninhibited by the presence of particulate pollutant material such as fly ash.
It is another object of the invention to provide such a method of removing pollutants by heat exchange in which particulate materials are prevented from negatively affecting the performance of the heat exchange step.
A further objective is to make commercially valuable byproducts from the condensate that is produced when the flue gas of fossil-fuel fired boilers is cooled.